Optical communication systems have been spread in order to realize large-capacity and high-speed information communications. Optical communication has been spreading not only for long-distance communication, but also as internal data communication method for a server and a supercomputer. In particular, optical interconnect for connecting boards or chips by optical wiring instead of the electric wiring in the past has been attracting attention. In an optical receiving circuit disposed at a reception-side front end of optical communication, a received optical signal is detected as an electric current by a light receiving element such as a photodiode. A trans-impedance amplifier (TIA) converts a signal of the detected electric current into a voltage signal, and amplifies the voltage signal to voltage amplitude suitable for analog-digital conversion in the next stage.
FIG. 1A illustrates a configuration of a general optical receiving circuit. A common-base amplifier is used as a preamplifier of the TIA. In the common-base amplifier, a noise-reduction effect is obtained by increasing a load Rc on a collector side. In FIG. 1, circuit noise is represented by Expression (1).
                                          i            conv            2                    _                =                                            i              bc              2                        _                    +                                                    v                a                2                            _                                      R              C              2                                +                                    i              a              2                        _                                              (        1        )            
A first term on the right side of Expression (1) represents a noise current in terms of input. A second term and a third term on the right side represent an input noise current of a post-stage amplifier. The second term represents a thermal noise component (current) from the load Rc, which is obtained by dividing a noise voltage (the square of a voltage) to the post-stage amplifier by the square of a resistance value of the load Rc. It is possible to reduce an input noise component of the second term by increasing the resistance value of the load Rc.
However, if optical input power, that is, an input current Ipd to the common-base amplifier is large, the load Rc with a large resistance value makes a potential Vcp3 in an output node so large that the post-stage amplifier may not normally operate. As illustrated in FIG. 1B, this problem is more conspicuous when the resistance value of the load Rc is larger. This is because a voltage drop in the load Rc decreases corresponding to the increase in the resistance value and the potential Vcp3 increases exceeding an appropriate range.
Since a rise in the potential Vcm during an input of large optical power may cause a failure of DC potential, there is known a configuration for switching the load resistors 214 and 215 together with the voltage sources Vdd4 and Vdd5 as illustrated in FIG. 1C in order to reduce such a failure of the DC potential (see, for example, Japanese Laid-open Patent Publication No. 2009-246823).
Meanwhile, there is known a configuration for increasing a bias current on an emitter side of the common-base amplifier when an input current from the photodiode increases, thereby keeping a dynamic range of a differential amplifier circuit unchanged (see, for example, Japanese Laid-open Patent Publication No. 2003-037453).
In addition, there is known a configuration for connecting a common-emitter amplifier circuit to an output of the common-base amplifier to form a negative feedback loop (see, for example, Japanese Laid-open Patent Publication No. 2014-116851).